As a new display technique, compared with a field effect Thin Film Transistor (TFT) Liquid Crystal Display (LCD), an Active Matrix Organic Light Emitting Diode (AMOLED) display has many advantages in the scope of viewing angle, picture quality, efficiency, and cost, and accordingly has a great potential for development in the field of display manufacture.
The AMOLED is driven by a current generated by a driving TFT in a saturation state and thus can emit lights. The luminance uniformity is always very poor because when inputting a same gray voltage, different critical voltages may generate different driving currents, resulting in that the constancy of the currents becomes very poor.
FIG. 1 illustrates a traditional 2T1C circuit comprising only two TFTs, wherein T1 is a switch transistor and DTFT is the driving transistor for the pixel circuit. The scan line Scan turns on the switch transistor T1, the data voltage Data charges or discharges the storage capacitor C; during the period of light-emitting, the switch transistor T1 is turned off, the driving transistor DTFT remains turned on by the voltage stored in the capacitor, and a turn-on current enables the OLED to emit lights. To implement a stable displaying, the OLED is required to be provided with a stable current. The advantages of a voltage-controlled circuit are that the structure is simple and it is rapid to charge the capacitor, but the disadvantage is that a linear control of the driving current is difficult, because the manufacture process for a Low Temperature Poly-Silicon causes the uniformity of the threshold voltages of the DTFTs very poor, at the same time the threshold voltage may also shift, and the threshold voltages of the different TFTs may be different greatly even though they are manufactured with the same process parameters, which leads to the problems of very poor luminance uniformity and luminance decay for the luminescence for driving the light-emitting circuit.